Inside Asteroid Family Trees

Asteroid origins can be hard to trace

- BY PHIL PLAIT

WE'VE ALL SEEN THIS HAPPEN in a science-fiction movie: our plucky heroes jump into their ramshackle spaceship and escape the bad guys by flying through the treacherous asteroid belt, where huge rocks tumble and spin so close together that the crew has to constantly dodge, duck, dip and dive to avoid being smashed to atoms.

It's exciting, but it's wrong: asteroids so close together would grind one another to dust in short order, making it extremely unlikely that you'd ever find such a situation near a star. In our solar system, the odds are pretty good that you could stand on the surface of an asteroid and not even be able to see another one! Big ones tend to be many millions of kilometers apart.

Yet they do interact if they are given enough time. Even in the sprawling main asteroid belt between the orbits of Mars and Jupiter, collisions are inevitable. In fact, we've managed to see some small asteroid smashups; bigger rocks are far more rare, so larger collisions are proportionally less common. But they do still happen—spacecraft reconnaissance of large asteroids shows that they are riddled with ancient impact craters. And when two space rocks go “bump” in the main belt, their high orbital speeds mean they can have collision velocities far higher than that of a rifle Shrapnel is inevitable because big impacts blow lots of asteroidal real estate out into space.

What happens to that ejected debris? In many cases, these fragments stay on much the same orbital path as the parent asteroid, although they gradually separate from it because of slight velocity differences. After millennia the ejecta might be clear across the sun from its source. You might think this outcome must be problematic for anyone trying to track down different types of asteroids to figure out how they all fit together—and it is! But this problem of orbital mechanics provides its own solution.